Optimize sending of a reply in RPC server

RPC server uses direct_send method to send replies.
 The method sets "passive" flag on an exchange.
 Thus auto_declare doesn't bring any good there, but it costs
 an additional "exchange.declare" call to RabbitMQ.
 simulator.py RPC tests shows that a perfomance gain is ~7%

Change-Id: Ia9f5f9f52c6cb324eca4ca6a63e80d604ddefa23

oslo_messaging/_drivers/impl_rabbit.py

@@ -974,6 +974,7 @@ class Connection(object):
         """Publish a message."""
         producer = kombu.messaging.Producer(exchange=exchange,
                                             channel=self.channel,
+                                            auto_declare=not exchange.passive,
                                             routing_key=routing_key)
 
         expiration = None